Improved control over the ignition and combustion characteristics of a fuel charge in an internal combustion (I.C.) engine has been a long sought goal. In diesel (compression ignition or C.I.) engines, problems associated with dependably igniting a typical diesel oil fuel are well known and have been extensively documented, particularly in connection with high speed automobile and truck diesel engines. Also extensively documented, particularly in recent times, are problems associated with smoke and particulate exhaust emissions which are also related to ignition characteristics of diesel fuel.
It is also recognized that alcohol fuels that might be regarded as appropriate, at least in a marginal sense, in conventional spark ignited (S.I.) engines, are regarded as difficult or inappropriate fuels for diesel engines due to their high heat of vaporization (resulting in excessive cooling effects in the combustion chambers) and their low cetane numbers (resulting in difficult or undependable compression ignition due to excessive ignition delay).
In the case of S.I. engines, it has been recognized that the combustion of gasoline type fuels at compression ratios conventionally used in modern engines is limited by the knocking tendency of the fuels. Antiknock additives, of course, are commonly used, as are alcohol blends to reduce the knock tendency of gasoline fuels. It is highly desirable to obtain clean, complete combustion of gasoline type fuels without knock at all operating regimes of S.I. engines.
Various approaches to improve ignition characteristics of fuels in diesel engines have met with mixed degrees of success, but the particulate emission problem with conventional diesel fuels and ignition problems with alcohol fuels remain difficult if not seemingly impossible to solve without substantial modification to the conventional diesel engine, and without substantial treatment of the fuel or exhaust stream.
Also, as indicated previously, various approaches have been taken to improve the antiknock characteristics of gasoline fuels for S.I. engines, all of which generally require additives to the fuel, which increases the cost of producing the fuel product.
On the other hand, interesting recent developments in the field of combustion technology, as well as certain older discoveries in combustion related disciplines, in particular, the importance of chemical activity leading up to the oxidation reaction of fuel substances in air at elevated temperatures and pressures, as well as of the physical environment needed for producing dependable spontaneous ignition of diesel fuels and knock-free combustion of gasoline fuels, have led to investigations by the inventors of the role of radical species of hydrocarbon liquid fuels in the complex process of ignition and combustion of fuels in C.I. and S.I. internal combustion engines.
The present invention arises from the recognition that controlled seeding of a fuel charge before ignition in a C.I. or S.I. engine with highly active radical species of fuel generated in a cool flame process (i.e., partial cool flame oxidation reaction) can produce dependable and predictable ignition and knock free combustion of fuels normally considered difficult to ignite without ignition improvers (in the case of a C.I. engine) or subject to knock during certain engine operating conditions (in the case of S.I. engines), due to the chemical conditioning of the compressed fuel charge. Indeed, it must be recognized that the entire process of ignition and combustion of a hydrocarbon fuel is a chemical exothermic reaction involving rapid oxidation of fuel to produce heat and expansion energy that is harnessed effectively to produce motive force. Any process that chemically optimizes the reaction will inherently improve the ignition and combustion characteristics of the fuel and improve engine operation and exhaust emission characteristics due to better and more complete combustion. Undue complication of the engine or its combustion chamber, or the handling of the fuel/air supply and the exhaust stream will also be avoided.
The problem is how to generate and manage the supply of radicals in the combustion chamber to achieve the recognized benefits that can be obtained from such seeding. Generation of radicals per se is relatively simple: heat an air and fuel mixture at elevated temperature and pressure so that it "cooks" or partially reacts in a cool flame oxidation process to produce various highly active radical species of the fuel and oxygen which will readily combine chemically with other molecules and radical species. However, what is complicated is introducing a suitable quantity of such radical species into a fuel charge within a closed combustion chamber in an engine in an efficient yet effective manner with minimum complexity and alteration of the existing engine and its combustion chamber. In the case of a C.I. engine, the required quantity of radicals is that population of radicals in a given fuel charge for a given engine that will produce a desired preselected ignition characteristic. For example, the characteristic may be dependable ignition timing of a low cetane fuel at a relatively low compression ratio, or it may be cleaner combustion of a higher cetane fuel with minimum smoke and particulate emissions. In the case of an S.I. engine, the required quantity of radicals is that population required to achieve complete, very rapid combustion of a fuel charge without premature ignition of end gases normally at the end regions of the combustion chamber reached lastly by the combustion flame front. As is well known, such premature ignition results in a sudden reaction producing a very rapid and often destructive pressure rise in the combustion chamber with audible noise known as "knock".
Various approaches taken in this regard are described in U.S. Pat. No. 4,002,151 granted Jan. 11, 1977 and U.S. Pat. No. 4,317,432 granted Mar. 2, 1982, both of which are incorporated herein by reference for their descriptions of problems to be solved in this field, the mechanisms and chemistry for radical generation by partial oxidation reaction of fuel and air, the composition of radicals resulting from such reactions, the influence of radicals as ignition centers in combustion of liquid fuels in internal combustion engines, and the relationship between the self-ignition point of fuels and the temperature and pressure conditions in the combustion zone (i.e., see FIG. 5 of U.S. Pat. No. 4,317,432).
Another approach to generating and managing radicals to improve combustion of hydrocarbon fuels (i.e., "radical enhanced combustion") is disclosed in U.S. Pat. No. 4,592,318 granted Jun. 3, 1986 and assigned to the assignee of this invention. This patent is also incorporated herein by reference for its discussion of the significance of radical seeding of a fuel charge and the influence of radicals on the autoignition point of fuels under variable temperature and pressure conditions as investigated and reported by N. N. Seminov (i.e. see FIG. 14 of the patent and the related discussion).
In this U.S. Pat. No. (4,592,318) it is recognized that fuel radical species can be generated in a controlled manner in a resonating chamber provided in the outer periphery of a piston, wherein the chamber is isolated from a main combustion chamber except for a critical gap or slot orifice that produces a resonating condition that pumps air into the main combustion zone, and which may also produce a choked flow of fluid from the resonating chamber into the combustion zone at the moment of opening of the exhaust valve at the end of the expansion portion of the cycle. The gap is also disclosed in the patent as providing a choked flow condition into the resonating chamber during the compression part of the cycle at least at higher engine operating speeds to thereby produce a variable compression ratio for the engine, dependent upon engine speed.
While the apparatus and process described in U.S. Pat. No. 4,592,318 achieved its intended purpose, namely clean, complete combustion of fuel without undesirable emissions, and while the apparatus reduced the knock tendency of engines incorporating the described combustion system, it has now been discovered that, for ceratin engines, dependable ignition and combustion characteristics can be achieved by utilizing a secondary chamber that communicates with the main combustion chamber through an orifice that is substantially choked at all operating speeds of the engine. Moreover, the importance of retaining radicals in the secondary chamber beyond the exhaust portion of the combustion cycle and the importance of providing a bowl or recess in the piston for containing most of the fuel of the charge was not recognized in the context of improving ignition characteristics of diesel fuels in C.I. engines and obtaining rapid, complete combustion without knocking in S.I. engines.